The present invention relates to an automatic vehicle location system for use in data communication, and more particulary, in a data communication system using a biphase shift keying (hereinafter, referred to as "BPSK"), to a BPSK modulation circuit supplying a higher output power with band limitation.
Referring to FIGS. 1A and 1B, in a known BPSK modulation system, data from a data generator 10, as shown by a waveform A, and a carrier wave from a local oscillator 12, as shown by a waveform B, are mixed together at a mixer 14 to produce a modulation signal for transmission.
Such signal can not have a wide transmission bandwidth because the frequency band is allotted to many other data transmissions. In order to improve such a drawback, a BPSK modulation method to limit the bandwidth of a transmission signal as shown by a waveform A of FIG. 2B is to filter data produced from the data generator 10 through a waveform shaping filter 11 to obtain the waveform B of the data as shown in FIG. 2B. The output of the waveform shaping filter 11 and a carrier wave produced from the local oscillator 12, as shown by a waveform C of FIG. 2B are mixed together through the mixer 14 to produce a signal D modulated as shown in FIG. 2B. Though this modulated signal has a limited bandwith according to the waveform shaping characteristics, there occur damped amplitude portions. Hence, in order to transmit the modulated signal, it must be highly increased in output power. Furthermore, when the modulated signal passes a power amplifier, the damped amplitude portions are not amplified because they are below the minimum input power, so that the waveform is distorted and does not accurately transmit the data.